It's been nine months since Google launched its Apps Marketplace. It gives third-party developers access to the same cloud platform as Google Apps for Business, which offers organizations tailored, private versions of Gmail, Google Docs, and other Google apps. It's all part of Google's push to get workers into cloud-based applications, and away from software such as Microsoft Office.

Some of the apps in the Marketplace complement existing Google Apps functionality, although most offer entirely new functionality, such as accounting or customer relationship management (CRM).

Because the apps are cloud-based they're usually accessed through a Web browser, rather than downloaded. The apps can be accessed on phones and tablet devices in addition to laptops and desktop computers. Additionally, cloud apps can be accessed anywhere there's a network connection.

A significant number of the apps are free, although licensing costs for the paid-for apps are perhaps slightly lower compared to that of traditional PC-based software, and are usually on a per-user, monthly basis.

[ For complete coverage of the Cloud Apps Wars -- including a complete guide to the business war, the competing products including Google
Docs and Office 2010, the implications for users and IT, and more -- see CIO.com's Cloud
Apps Wars Bible. ]

The nine months since the Marketplace inception have clearly been busy as there are now more than 250 applications available, and four million users have at least one of those apps installed on their Google Apps domain.

As the year draws to a close Google has put together a top 10 list of user favorites. The three most popular categories are Project Management, Customer Relationship Management (CRM), and Accounting and Finance. Of those, Google says CRM is the most searched-for term by those looking for new apps. It perhaps comes as no surprise that the top 10 list is populated by either free apps, or ones that are cut-down versions of premium applications.

The top 10 is as follows, with each entry listed alongside their category and upgrade fees, if any.

This project and task management tool offers total integration with Google Docs, such as the capability to attach Docs to tasks, projects and events, and to assign tasks via e-mails. It integrates with existing Google Calendars too, to give an overview of project milestones--and it features a microblogging feature.

This CRM and project manager features clever e-mail integration that creates new client details based on sent and received messages. The app will automatically search online to find details about clients for you, and it offers an at-a-glance representation of current events.

This presentations and design suite with built-in image and audio editing can be used for everything from creating slides to mixing podcasts. A screen capture tool helps with creating presentations. A built-in vector editor can even create T-shirts and icons.

This project management service integrates tightly into Google Docs, and offers the capability to invite clients to view workspaces. Milestones, deliverables and tasks are automatically added to Google Calendar too.

This accounting and tax app lets you share access with your accountant. It ties in with online banks, so can automatically pull-in data. Customer payment histories can be viewed within Gmail, and the app integrates with Google Calendar, so you can see when payments are due.

This e-mail campaign and newsletter-sending app touts powerful reporting tools, such as tracking how many of the e-mails are opened and by whom. IT features an HTML e-mail-creation tool, and can integrate with social networking tools includingTwitter and Facebook.

Free for up to 1,000 subscribers and up to 6,000 emails per month; various pricing tiers from $15 up to $240 per month for 50,000 subscribers

This simple project management tool is built around To Do lists, featuring Gmail integration to show who's assigned to a task. Private or personal task lists can be created, and tasks assigned to various users. Due dates are automatically added to Google Calendar.

Free for up to five user assignations and 100MB of storage; additional packages adding five more user assignations and 1GB of storage available at $5 per month

Insync is similar to services like DropBox in that client software must be downloaded (PC or Mac), which creates a folder on your hard disk that syncs automatically online. Anything placed in the special "Google Docs" folder is automatically imported into that service, and any items created within Google Docs are automatically saved as files.

Free for five users and 1GB of storage shared amongst them; further price plans TBA

This Enterprise resource planning (ERP) service comes with a comprehensive list of features, including CRM, invoicing and billing creation, project management, and accounting tools. It integrates into Gmail and Google Apps contacts.